There has been heretofore disclosed a technique that, in the case where the reading and writing of information is carried out with respect to optical disks having different standards for the track pitch on the information recording surface, electrodes respectively having two different electrode patterns are used, then voltages of different space patterns are applied to a liquid crystal, and different diffraction gratings are formed, in order to avoid providing a plurality of diffraction gratings in an optical head apparatus (for example, refer to references 1 and 2).
In the case of the liquid crystal diffracting device disclosed in the reference 1, transparent electrodes 73a, 73b each having a different electrode pattern formed on a transparent substrate 71 are used by switching the electrodes one another. A voltage is then applied to either the transparent electrode 73a or 73b and to a transparent electrode 74 formed on a transparent substrate 72 as oppositely disposed to the transparent electrodes. In this way, a different voltage of the space pattern is made applied to a liquid crystal 75. Incidentally, the transparent electrode 73a is insulated from the transparent electrode 73b. The switching for the transparent electrodes 73a and 73b is carried out by an applied-voltage switching means 76. As a result, the liquid crystal diffracting device realizes diffraction gratings corresponding to the electrode pattern of either the transparent electrode 73a or 73b to which the voltage is applied by the applied-voltage switching means 76.
On the other hand, a diffraction grating disclosed in the reference 2 has two liquid crystal display units. A stripe shaped transparent electrode is formed on each of the liquid crystal display units. The voltage applied liquid crystal display unit is functioned as a diffraction grating. The non-voltage applied liquid crystal display unit is made functioned as a transparent flat plate. The liquid crystal display unit to be applied a voltage is switched so that the diffraction grating is made switched.
Further, a diffraction grating disclosed in a reference 3 includes two transparent substrates formed of electrode patterns alternately configured one another, and a liquid crystal layer held between the transparent substrates. A voltage is switched to one another and applied to one of electrodes, as a set, alternately configured one another, so that a different voltage of space pattern is applied to the liquid crystal layer, and a diffraction grating pattern is switched.
[Reference 1] JP-A-4-283430
[Reference 2] JP-A-9-282684
[Reference 3] JP-A-2006-99947
However, in the case of the optical head apparatus provided with liquid crystal diffraction gratings of such a related art, there are two electrodes in which one is applied with a voltage and the other is not (for example, transparent electrodes 73a and 73b in the constitution shown in FIG. 7). In this case, there has been a problem that the liquid crystal diffraction gratings are susceptible to the static electricity etc. since the electrode not being applied with any voltage is electrically floated, characteristic variation caused by the time lapse occurs easily. Further, there has been problem that the liquid crystal diffraction gratings as related art requires at least three terminals used for applying voltages, so that a driving circuit is made complicated and cost-cutting is limited.